1. Field of Invention
This invention concerns a reactor for heterogeneous synthesis under pressure, and more particularly for the catalytic synthesis of ammonia (from nitrogen and hydrogen) and methanol (from carbon monoxide and hydrogen), said reactor involving the use of a granular catalyst (in various shapes and with different specifications) arranged in one or more superimposed layers, with the gas running through each layer in a first zone with a prevalently axial flow and in a second zone with a prevalently radial flow (down-flow axial-radial reactor with gas running downwards) or vice versa (up-flow radial-axial reactor with gas flowing upwards; first zone with prevalently radial flow and second zone with prevalently axial flow).
2. Statement of the Prior Art
The problems affecting synthesis reactors are well known, particularly when it is necessary to use a considerable volume of catalyst (low-pressure and high-capacity ammonia and methanol plants). To contain pressure drops from the catalytic bed, and thus energy consumption, axial flow reactors have become very wide and this limits their capacity and increases their cost (for example, ICI reactors for ammonia and methanol). To overcome this inconvenience, radial flow reactors (for example U.S. Pat. No. 4,181,701, Topsoe) have several catalyst layers with circular crown sections and each layer must be sealed at both ends (sealing baffles). This involves burdensome construction to avoid the problems arising from the expansion of the materials used for the various internal parts of the reactor, and further complications when loading and unloading the catalyst. According to this known technique, the catalyst layers are arranged in a very complex single metal structure (catalyst basket) situated inside the reactor's shell; burdensome equipment is usually required to lift this structure for maintenance and for replacing the catalyst.
On the other hand, the various synthesis loops currently used in ammonia production are all based on the same process scheme, so that the different technologies are fundamentally characterised by reactor design and by the scheme for recovery of heat produced in synthesis. The internal parts of the reactor (cartridge) are designed to minimize gas pressure drops, while ensuring better gas distribution through the catalytic beds and facilitating the introduction of exchangers for exchanging heat between reacted and fresh gas. The design of the reactor must also ensure ease of access for maintenance and for loading and unloading the catalyst. According to the recent low-energy process schemes using low-pressure loops in large reactors, the above-mentioned requirements become even more critical, since larger amounts of recycle gas are involved.
The most widely used reactors are arranged vertically with axial gas flow (Uhde-ICI-Kellogg) or radial gas flow (Topsoe), with the exception of a single horizontal reactor (Kellogg) installed in a large production plant (Japan).
Similarly to the external shell the cartridge, i.e. the internal part of the reactor, is usually made in a single piece, requiring considerable effort at the construction stage and during transport, erection and maintenance, particularly in large production plants. In conventional reactors with shell and cartridge in a single piece gas flow can be either radial or axial; radial gas flow (Lummus, Topsoe, Kellogg; U.S. Pat. Nos. 3,918,918 and 4,181,701, European Patent Application No. 0,007,743-1) seems the most suitable for large reactors in low-pressure plants.
In axial-flow reactors it is imperative to use large-size catalysts in order to contain pressure drops, thus increasing the specific volume of the reactor.